A device of this type would include a radio frequency transponder. Such a device transmits a reply signal upon reception of an incoming signal. It usually includes a receiver, responsive to the incoming signal, for providing an amplified intermediate signal and a transmitter, responsive to the amplified intermediate signal, for transmitting the reply signal.
For instance, present day radio frequency transponders are used to identify objects, possibly along with the contents thereof and even the location. An interrogator is used to send a radio frequency signal to the transponder, where the transponder will reply back, potentially with information, also with a radio frequency signal. Such transponders can be self-powered, enabling them to be mobile. Transponders are usually required to work at ranges that make it impractical to provide power by means of a radiated electric or magnetic field. The power supplies that are used to power the transponders are limited, meaning they contain or are able to supply only a limited amount of energy. Typically batteries are used but other sources could be used such as capacitors charged by solar or mechanical means, pressurized or flammable gas, etc. The consumption rate is proportional to the amount of time the transponder is powered to be operational.
These days it is becoming more and more important to run such devices with increased efficiency. Power consumption can be managed by having various modes, e.g., a standby (low power consumption), sleep (minimized power consumption) or reply mode (high power consumption). It is a desirable feature to have the transponder be available as much as possible, work at a long range but consume little power. These are conflicting requirements and tradeoffs are made to compromise one feature for the other. A way to increase battery life of a transponder would be to operate it with an on/off ratio, thereby making the transponder available only periodically. This would reduce the average power consumption at the cost of availability. In certain cases the power consumption and availability also become conflicting requirements.
To reliably cover the range between the interrogator and the transponder, a certain amount of power is required to develop signals of adequate levels to propagate energy between the units. As the range increases, power as seen by the receiving unit is reduced.
An increase of transmit power of the interrogator or receive sensitivity of the transponder will increase range capability to the transponder. This is accomplished at the expense of consuming more input power from a power source such as the above-mentioned battery.
A comparable amount of increase in range must be considered for both the interrogator and transponder since the path being used is two-way or bidirectional. Simply increasing the performance in one direction would provide no benefit. Providing more power to the interrogation signal to increase the range to the transponder will also provide no benefit if the interrogator is not sensitive enough to receive the transponder reply or the transponder is not powerful enough to respond back.
While excess battery power may be available at the interrogator, it may not be at the transponder. The transmit or reply section of the transponder typically can have its transmission time minimized to conserve power. The transponder reply could be a transmit burst that occurs when an interrogator requests a reply. However, the transmit burst may not be able to be reduced further in time without loss of transmitted information.
In a system where the interrogator could randomly interrogate the transponder, it would be required that the transponder receiver portion be operational and highly available for reliable operation. This may require that the transponder receiver be operational for much longer periods of time than the transponder reply section.
In an example, if the transponder is requested to reply 200 times per day, and transmits a burst of information with a duration of 10 milliseconds each time, the total transmit time per day would equal 2 seconds. If the receiver section is powered continuously, the ratio of receive time to transmit time is over 43,000:1. This large ratio shows that significant changes would have to be made in the manner that the transmit section consumes power for it to become the dominant power consuming source.
Therefore the only way to reduce the power consumption of the transponder is to increase its operating efficiency, and a need exists to significantly increase the operating efficiency of a receive section in a transponder. An increase in the operating efficiency of a transmit section in a transponder would also be beneficial. Such an advance could benefit radio frequency circuitry in general and could be applied to any kind of radio frequency circuitry including receiver, transmitters, and various signal-processing stages. Transponders have in the past used amplifiers in both the receiver and transmitter sections thereof. These amplifiers can consume most of the power needed to operate the unit.
The prior art approach using conventional amplifier technology will now be described. It would be desirable to achieve e.g. 80 dB gain with low power consumption. For conventional technology using amplifiers to achieve the 80 decibels of signal gain would require a power consumption of about 50 milliwatts per 20 decibels of gain, which translates to a current consumption of about 17 milliamps at a working voltage of 3 volts. See for instance a Mini-Circuits MAR6SM that uses 56 milliwatts while providing 20 decibels of gain (Mini-Circuits, P.O. Box 350166, Brooklyn, N.Y. 11235 USA; http://www.minicircuits.com). The total current consumption for the full 80 decibels of gain would be about four times the above-mentioned current consumption of 17 mA (for the 20 decibel gain amplifier), or 68 milliamps. For a limited power supply that only has an energy capacity of e.g. 170 milliamp hours (AA battery), this means that the conventional technology will run for about 170 maH/68 ma or for about 2.5 hours. Using the 170 maH power source and requiring for instance a receiver to run for a much longer period, e.g. a targeted 6 months, will limit the current consumption to an average current of about four microamps. This is about 17000 times less current consumption than what is possible with conventional technology.
Conventional super heterodyne receivers contain a local oscillator circuit that is crystal controlled and may take 10 milliseconds just to turn on. The cumulative turn-on time first needs to be considered, ignoring for the moment the need to run the receiver for a period of time after turn-on in order to perform its receiver function. To do this, a calculation is made to see how many 10 millisecond turn-on times can be performed in the targeted six month period. Each time the receiver is powered, it consumes 68 milliamps. Therefore the receiver can be turned on for (170/68)*(3600/10 ms)=900,000 times. Over a six month period, this would allow the receiver to operate about every 17.3 seconds. This is greater than the desired availability of every 5 seconds and only considers the turn-on time. Still only considering the turn-on time, if the device is turned on every 5 seconds for 10 ms, the original duration or goal of the battery being able to supply power for the sixth month period gets reduced to about 52 days. Looking at it a different way, becoming available every 5 seconds with a power consuming duration of 10 milliseconds each time, presents a duty cycle of 500 to one. Taking the original continuously powered receiver that lasted for 2.5 hours and extending the battery life by the accepted unavailability of the 500: 1 duty cycle will make the battery now last 2.5*500 or 1250 hours or about 52 days. This is shy of the target of 180 days or 6 months. This calculation also does not take into account leaving the receiver circuit on for some selected time after it turns on and becomes operational to actually receive the signal and detect any information.
Also, assuming a transponder application, a transmitter section or responding part of the device will consume additional power. An intelligent power control can be used to minimize the additional power required by activating this portion of the circuitry only when necessary. In an example, if this section of the circuitry is activated to respond every 5 seconds and consumes an additional 10 milliamps of current (in addition to the receive section) for an additional 10 millisecond duration, the additional load on the limited power source reduces the battery life to about one half the life as compared to just powering the receive section. This would reduce the battery life from 52 days to less than 26 days. It is therefore important to intelligently control the time during which the response section is activated. If the response section is activated only when required, in an example one hour per day, this only will increase the power consumption a few percent allowing the battery life to be a few percent less than the 52 day period or about 50 days.
Known transponders may use crystal detector receivers to receive a radio frequency interrogation signal modulated by an audio frequency signal and use audio amplification circuits for detection purposes. However, it such a circuit, the receive sensitivity is limited by the crystal detector sensitivity of approximately −50 dBm so the range is limited for a given interrogation power level. To get better sensitivity, other transponders use RF gain stages that are more sensitive but require more power. These devices are known to use power management but they inherently consume a large amount of power to obtain a higher degree of availability and sensitivity.
Synchronous oscillators (SOs) are known generally for instance from US 2003/0011438 A1 published Jan. 16, 2003 by Vasil Uzunoglu There, a modification of the synchronous oscillator is described, having regenerative positive feedback. The circuit includes an amplifier, a high-Q tank circuit, and a conventional synchronous oscillator feedback network. An additional feedback path provides a negative impedance conversion effect, according to Uzunoglu. There are various articles and US Patents by the same inventor Vasil Uzunoglu, (U.S. Pat. Nos. 4,335,404, 4,274,067, 4,356,456) relating to SOs (see the list at page 15 of US 2003/0011438). Various applications of the modified SO are shown. A further important characteristic of the synchronous oscillator is its energy efficiency. According to Uzunoglu, the regenerative feedback results in very little power dissipation, enabling the circuit to operate highly effectively with very low power supply requirements, for example, approximately 2–3 volts.
The above-mentioned importance of running radio frequency devices with increased efficiency is particularly important for portable devices. If a constantly powered synchronous oscillator were to be used in a portable radar or radio device with a limited power supply such as a battery, the limited power supply would run down rather quickly. For instance, two AA batteries in series (3 volts) only have approximately 170 milliAmpere-Hours of total energy available for a load such as an S.O. (e.g., 1 mA load) and will therefore last only for a week or so. Nonetheless, it would be desirable that the device e.g. a receiver be able to operate with batteries that are widely available, such as AA size alkaline types and have the batteries last at least for a six month period in a receive mode. It would also be desirable to have the receiver work continuously or be available often and very quickly. Such a receiver should be sensitive enough to receive an interrogator signal transmitted from an interrogator transmitter at a selected power, e.g., 1 milliwatt at a selected distance, e.g., of up to 150 meters at a frequency of around 950 MHz, for instance. In such an example, the transmit path length and interrogator power level will require the receiver sensitivity to be about −80 decibels below the 1 milliwatt power level because approximately that amount of power is lost in the path length alone (assuming the transmit signal from the interrogator is dissipated in all directions at once). For a robust receiver design it is desirable to have excess sensitivity, high availability, and minimized power consumption. Higher receiver gain will of course consume a proportionately higher amount of power.